1. Field of the Invention
The present invention relates to an integrated circuit (IC) for controlling a switching power supply.
2. Description of the Background Art
One example of recent efforts to reduce the burden on the environment is reducing the standby power consumption of electronic products. Switching power supplies, which convert a commercial power supply to a direct current in order to supply power to electronic products, are also no exception. And because there are many products to which timers and remote controllers are added, reducing power consumption during standby is becoming particularly urgent.
In a switching power supply having a relatively low output power, standby power consumption can be reduced by lowering the switching frequency and reducing switching loss during standby. Further, in such a relatively low power output switching power supply, a power supply for a normal load and a power supply for standby (sub-power supply) can be provided, and standby power consumption can be reduced by switching between the power supplies and causing them to operate in response to the state of the load.
One way of reducing standby power consumption with a single switching power supply, without using a sub-power supply, is a method using a switching power supply control IC having two modes—a normal operating mode and a standby operating mode. With a dedicated terminal, it is possible to switch between these two modes with a control signal external to the IC. In some cases where the external switching control signal is generated with a power supply device, and in other cases a signal from a microcomputer or the like is utilized (e.g., see Japanese Patent No. 2,956,681).
FIG. 5 shows an example of a control IC circuit having a dedicated terminal to which an external switching control signal is inputted, and which can be used as a control circuit of the converter system disclosed in Japanese Patent Application Publication (JP-A) No. 2002-209381, which employs a switching method that is the same as that in the conventional example described in Japanese Patent No. 2,956,681.
In the control IC shown in FIG. 5, continuous switching is implemented in the normal operating mode, and burst switching using a burst frequency and a burst ON period created inside the control IC is implemented in the standby operating mode. Further, burst switching is an operation that repeatedly starts and stops high-frequency switching at a burst frequency that is lower than the frequency of the high-frequency switching operation. This will be described with reference to FIG. 5.
The double circles shown in FIG. 5 represent terminals of the control IC. FB, CB and STB at the left side of FIG. 5 are input terminals, and OUT at the right side of FIG. 5 is an output terminal that drives the gate of a power MOSFET. Ordinarily, a photodiode or a phototransistor is connected with respect to GND (ground), and a return signal corresponding to the output voltage of the power supply is inputted to the FB terminal. This signal and an oscillation waveform that is an internal oscillation circuit (OSC) output are inputted to a comparator represented by the PWM portion, whereby a pulse having a width corresponding to the splitting of each signal is generated. This pulse becomes an output pulse for conducting pulse width modulation (PWM) control where the pulse width changes due to the return signal corresponding to the output voltage of the power supply.
A capacitor for determining the burst oscillation frequency in the standby operating mode is connected to the CB terminal. A burst circuit BURSTOSC switches between a constant sink/source current in accordance with the terminal voltage and outputs this to the CB terminal. The sink/source current is outputted with respect to the capacitor connected to the CB terminal, whereby triangle wave oscillation is conducted. The comparator represented by the OnTB portion compares the FB terminal voltage and the CB terminal voltage in the same manner that the PWM control pulse is generated, and generates a pulse with a width corresponding to the splitting of both voltages. The frequency of this pulse becomes the burst frequency, and the pulse width becomes the burst ON period.
The STB terminal is an input terminal for switching the operating modes of the control IC with an external signal, and inputs one of a High and a Low voltage. When the STB terminal voltage is High, the output signal of the OnTB portion is not transmitted from the next stage on by the OR circuit to which the STB terminal voltage is inputted. Thus, a pulse that is the same as the PWM portion output is outputted to the OUT terminal. This condition is called the normal operating mode, and the switching frequency at this time (i.e., the oscillation frequency of the circuit OSC) is f_OSC.
When the STB terminal voltage is Low, the OR circuit causes the output signal of the OnTB portion to be transmitted from the next stage on. Thus, AND outputs of the PWM portion and the OnTB portion appear at the OUT terminal. Here, assuming that f_OSCB represents the oscillation frequency determined by the circuit BURSTOSC and the CB terminal capacitor, f_OSCB is the burst frequency and is set to be about 1/100 or less with respect to f_OSC. Performing the AND operation between the high frequency pulse and the low frequency pulse in this manner implements burst switching.
As shown in FIG. 6, the circuit OSC comprises comparators CP1 and CP2, a reset/set flip-flop RSFF, inverters I1 and I2, two constant current circuits Ict, two switches SW, and a capacitor C. The circuit OSC uses the comparators CP1 and CP2 to compare the output voltage Vct with a predetermined set value, charge or discharge the capacitor C in accordance with the result, and output the triangle wave signal that is illustrated.
The circuit BURSTOSC is configured in exactly the same manner as the circuit OSC shown in FIG. 6. However, because their circuit constants are mutually different (e.g., the switching frequency f_OSC is higher than the burst frequency f_OSCB), the OSC current Ict is made larger than the Ict of the BURSTOSC so that the external capacitor can be quickly charged/discharged. That which finally determines the frequency is the capacitance value of the external capacitor.
In the circuit shown in, FIG. 5, it is necessary to provide a signal generating circuit inside the switching power supply to input a signal from outside of the control IC in order to switch between the normal operating mode and the standby operating mode. Even when the signal generating circuit is outside the switching power supply, it is necessary to provide a circuit for receiving the signal inside the control IC. Thus, there is the problem that the number of parts of the device itself is increased.